1. Field of the Invention
The present invention relates to stable pigment dispersions and to their use in colored layers, inkjet inks, and printing inks.
2. Description of the Related Art
Pigment dispersions are made using a dispersant. A dispersant is a substance for promoting the formation and stabilization of a dispersion of pigment particles in a dispersion medium. Dispersants are generally surface-active materials having an anionic, cationic, or non-ionic structure. The presence of a dispersant substantially reduces the dispersing energy required. Dispersed pigment particles may have a tendency to re-agglomerate after the dispersing operation, due to mutual attraction forces. The use of dispersants also counteracts the re-agglomeration tendency of the pigment particles.
The dispersant has to meet particularly high requirements when used for dispersing pigments. Inadequate dispersing manifests itself as increased viscosity in liquid systems, loss of brilliance, and hue shifts. Particularly good dispersion of the pigment particles is required in the case of inks for use in inkjet printers so as to ensure unimpeded passage of the pigment particles through the nozzles of the print head which are usually only a few micrometers in diameter. In addition, pigment particle agglomeration and the associated blockage of the printer nozzles has to be avoided in the standby periods of the printer.
Polymeric dispersants contain in one part of the molecule so-called anchor groups, which adsorb onto the pigments to be dispersed. In a spatially separate part of the molecule, polymeric dispersants have a polymer chain which sticks out and whereby pigment particles are made compatible with the dispersion medium, i.e., stabilized.
The properties of polymeric dispersants depend on both the nature of the monomers and their distribution in the polymer. Polymeric dispersants obtained by randomly polymerizing monomers (e.g., monomers A and B polymerized into ABBAABAB) or by polymerizing alternating monomers (e.g., monomers A and B polymerized into ABABABAB) generally result in a poor dispersion stability. Improvements in dispersion stability have been obtained using graft copolymer and block copolymer dispersants.
Graft copolymer dispersants consist of a polymeric backbone with side chains attached to the backbone. CA 2157361 (DU PONT) discloses pigment dispersions made by using a graft copolymer dispersant with a hydrophobic polymeric backbone and hydrophilic side chains. Other graft copolymer dispersants are disclosed in U.S. Pat. No. 6,652,634 (LEXMARK), EP 1182218 A (DU PONT), and U.S. 2004/0102541 (LEXMARK).
Block copolymer dispersants having hydrophobic and hydrophilic blocks have also been disclosed. EP 996689 A (DU PONT) discloses an AB block copolymer dispersant with a polymeric A segment of polymerized glycidyl(meth)acrylate monomers reacted with an aromatic or aliphatic carboxylic acid, and a polymeric B segment of polymerized alkyl(meth)acrylate monomers or hydroxyalkyl(meth)acrylate monomers. U.S. Pat. No. 6,413,306 (DU PONT) discloses ABC block copolymer dispersants with a polymeric A segment of polymerized alkyl(meth)acrylate, aryl(meth)acrylate or cyclo alkyl(meth)acrylate, a polymeric B segment of polymerized alkyl amino alkyl(meth)acrylate monomers with a quaternized alkyl group, and a polymeric C segment of polymerized hydroxyalkyl(meth)acrylate monomers.
U.S. Pat. No. 5,859,113 (DU PONT) discloses a pigment dispersion useful for forming coating compositions containing dispersed pigment, a carrier liquid and an AB-block polymer dispersant (binder); the AB block polymer has a number average molecular weight of about 5,000-20,000 and contains 20-80% by weight of a polymeric A segment and correspondingly 80-20% by weight of a polymeric B segment; wherein the polymeric A segment of the block polymer is of polymerized glycidyl(meth)acrylate monomers reacted with an acid from the group of aromatic carboxylic acids or aliphatic carboxylic acids; and the B segment is of polymerized alkyl(meth)acrylate monomers having 1-12 carbon atoms in the alkyl group, hydroxy alkyl (meth)acrylate monomers having about 1-4 carbon atoms in the alkyl group; and wherein the weight ratio of pigment to binder in the dispersion is about 1/100-200/100.
The method of group transfer polymerization (GTP) used for synthesis of AB blockcopolymers is disclosed by SPINELLI, Harry J. GTP and its use in water based pigment dispersants and emulsion stabilizers, Proc. of 20th Int. Conf. Org. Coat. Sci. Technol., New Platz, N.Y.: State Univ. N.Y., Inst. Mater. Sci. pp. 511-518. Other techniques include atom transfer radical polymerization (ATRP), RAFT (reversible addition-fragmentation chain transfer polymerization), MADIX (reversible addition-fragmentation chain transfer process, using a transfer active xanthate), catalytic chain transfer (e.g., using cobalt complexes), or nitroxide (e.g., TEMPO) mediated polymerizations.
The design of polymeric dispersants for ink-jet inks is discussed in SPINELLI, Harry J., Polymeric Dispersants in Ink Jet Technology, Advanced Materials, 1998, vol. 10, no. 15, pp. 1215-1218.
A wide variety of polymeric dispersants has been proposed, but the dispersion stability of pigments, especially in inkjets, still needs further improvement.